Uniaxially oriented polypropylene-based film

ABSTRACT

To provide a uniaxially oriented polypropylene-based film excellent in tensile elongation along the TD direction and transparence, and generating less fibers when it is torn, a film-forming material which comprises a polypropylene-based resin material consisting of 40 to 95% by weight of crystalline polypropylene, and 60 to 5% by weight of propylene-α-olefin copolymer dispersed as particles in the crystalline polypropylene is uniaxially oriented to afford a uniaxially oriented polypropylene-based film, wherein the particles of the copolymer have an aspect ratio (L/D) of mean dispersed particle length (L) to mean dispersed particle diameter along the film thickness direction (D) of 100 or more in a cross section of the film along the MD direction, and the mean dispersed particle diameter of 0.10 μm or less.

TECHNICAL FIELD

The present invention relates to a uniaxially oriented film formed froma film-forming material comprising a polypropylene-based resin material.More precisely, it relates to a uniaxially oriented polypropylene-basedfilm in which tensile elongation along the transverse direction isimproved without degrading transparence.

BACKGROUND ART

Uniaxially oriented polypropylene-based films have a property that theyare likely to be torn along the film-flowing direction during the filmproduction, i.e., the longitudinal direction called machine direction(MD direction), and thus they are excellent in cuttability along thatdirection (straight cuttability). Therefore, they are widely used mainlyin the field of food packaging, for example, individual packaging ofproducts likely to break, such as confectionery, individual packaging ofrice balls and the like.

While conventional uniaxially oriented polypropylene-based films areexcellent in the straight cuttability along the MD direction, however,they are poor in elongation along the perpendicular direction to the MDdirection (TD direction), and therefore they are likely to be torn whenforce is applied along the TD direction. Therefore, the films may bebroken when used for packaging of heavy contents.

Further, when the conventional uniaxially oriented polypropylene-basedfilms are torn along the MD direction, they generate fine fibers ontheir torn surfaces, which may be mixed in foodstuffs and the like.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a uniaxially orientedpolypropylene-based film in which tensile elongation along the TDdirection is improved without degrading transparence of the film, andwhich generates less fibers when it is torn.

The present inventors earnestly conducted studies in order to achievethe foregoing object. As a result, they found that, in a uniaxiallyoriented film composed of a polypropylene-based resin materialcomprising a propylene-α-olefin copolymer dispersed as particles incrystalline polypropylene, the straight cuttability along the MDdirection and the tensile elongation along the TD direction could beimproved without degrading the transparence by controlling form of thefilm so that the film should have a specific dispersion state of thecopolymer particles in a cross section along the MD direction of thefilm. Thus, they accomplished the present invention.

That is, the present invention provides a uniaxially orientedpolypropylene-based film formed from a film-forming material whichcomprises a polypropylene-based resin material consisting of 40 to 95%by weight of crystalline polypropylene, and 60 to 5% by weight ofpropylene-α-olefin copolymer dispersed as particles in the crystallinepolypropylene, wherein the particles of the copolymer have an aspectratio (L/D) of mean dispersed particle length (L) to mean dispersedparticle diameter along the film thickness direction (D) of 100 or morein a cross section along the MD direction of the film, and the meandispersed particle diameter of 0.10 μm or less.

The present invention also provides a uniaxially orientedpolypropylene-based film formed from a film-forming material whichcomprises a polypropylene-based resin material consisting of 40 to 95%by weight of crystalline polypropylene, and 60 to 5% by weight ofpropylene-α-olefin copolymer, and has a ratio of MFR of the crystallinepolypropylene to that of the propylene-α-olefin copolymer (MFR of thecrystalline polypropylene/MFR of the propylene-α-olefin copolymer) of 10or less, wherein the film is uniaxially oriented along the MD directionso that the orienting ratio should be 3 to 12 times.

The uniaxially oriented film of the present invention comprises thecopolymer particles dispersed elongatedly in a matrix of the crystallinepolypropylene in such a manner the particles should have an aspect ratiohigher than a certain level, and an oriented film composed of apolyolefin-based resin material having such a dispersion state has beenmade by the present invention for the first time.

According to the present invention, there can be obtained an orientedfilm that exhibits good straight cuttability along the MD direction,excellent tensile elongation along the TD direction and hightransparence, and does not generate fibers, thanks to such acharacteristic in form.

The production method of the film is not particularly limited so long asan oriented film having such a characteristic in form as mentioned abovecan be provided. However, a uniaxially oriented polypropylene-based filmwhich is composed of a polypropylene-based resin material comprisingcrystalline polypropylene and propylene-α-olefin copolymer and having aratio of MFR of the crystalline polypropylene to that of thepropylene-α-olefin copolymer of 10 or less, and oriented under a certaincondition can have the aforementioned characteristic.

The uniaxially oriented film of the present invention is useful as afilm for package, in particular, as a film for packaging heavy contentsor for food package.

Preferred embodiments of the present invention will be explainedhereinafter.

(1) Polypropylene-based Resin Material of the Present Invention

The film-forming material for forming the uniaxially oriented film ofthe present invention comprises a polypropylene-based resin materialthat consists of crystalline polypropylene and propylene-α-olefincopolymer, the copolymer being dispersed as particles in the crystallinepolypropylene (the copolymer is dispersed as domains in a matrix of thecrystalline polypropylene).

(i) Crystalline polypropylene

The crystalline polypropylene used for the present invention is acrystalline polymer comprising principally of propylene units, andpreferably comprises 90% by weight or more of the propylene units basedon the whole polymer. Specifically, it may be a homopolymer ofpropylene, or it may be a random copolymer or a block copolymercomprising 90% by weight or more of propylene units and less than 10% byweight of α-olefin. When it is a copolymer, the α-olefin may includeethylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene,4-methyl-1-pentene, 3-methyl-1-pentenel, and the like. It is preferablein view of the production cost to use a propylene homopolymer orpropylene-ethylene copolymer having a propylene unit content of 90% byweight or more.

The melt flow rate (abbreviated as “MFR” hereinafter) of the crystallinepolypropylene is preferably in the range of 0.1-50 g/10 minutes in viewof the stability upon film-forming.

(ii) Propylene-α-olefin copolymer

The propylene-α-olefin copolymer used for the present invention is arandom copolymer of propylene and an α-olefin other than propylene. Thecontent of propylene unit is preferably in the range of 20-80% byweight, more preferably 20-75% by weight, particularly preferably 20-70%by weight based on the whole copolymer. When the content of propyleneunit exceeds 80%, the desired dispersed state of the copolymer particles(referred to as “copolymer domains” hereinafter) in the matrix ofcrystalline polypropylene may not be obtained, and hence the improvementof the tensile elongation along the TD direction targeted in the presentinvention may not be obtained. On the other hand, when it is less than20%, the copolymer domains are difficult to be formed, and thus thedesired performance may not be obtained.

As the α-olefin other than propylene, ethylene, 1-butene, 1-pentene,1-hexene, 1-octene, 1-decene, 1-dodecene, 4-methyl-1-pentene,3-methyl-1-pentene and the like can be mentioned. Among these, apropylene-ethylene copolymer containing ethylene as the α-olefin ispreferably used because it is beneficial to the production cost.

While MFR of the propylene-α-olefin copolymer used for the presentinvention is not particularly limited, it is preferably in the range of0.1-20 g/10 minutes.

More preferably, MFR of the propylene-α-olefin copolymer is preferablyselected so that its ratio to MFR of the crystalline polypropylene (MFRof the crystalline polypropylene/MFR of the propylene-α-olefincopolymer, referred to as “MFR ratio” hereinafter) should be 10 or less,more preferably fall within the range of 0.1-5.

(iii) Polypropylene-based resin material

In the polypropylene-based resin material of the present invention, thecontent of the crystalline polypropylene is 40-95% by weight, preferably50-95% by weight, and the content of the propylene-α-olefin copolymer is60-5% by weight, preferably 50-5% by weight based on the wholepolypropylene-based resin material. When the ratio of the copolymer isless than 5% by weight, satisfactory tensile elongation along the TDdirection cannot be obtained. When it exceeds 60%, the rigidity of thefilm is markedly decreased, and it is not preferred for practical use.

The production method of the polypropylene-based resin material is notparticularly limited, and it can be obtained by any kind of method. Forexample, it can be obtained by mixing crystalline polypropylene andpropylene-α-olefin copolymer, which were polymerized separately, throughmelt-kneading or the like. Alternatively, it can be obtained bycontinuously polymerizing crystalline polypropylene andpropylene-α-olefin copolymer by multi-step polymerization.

Specifically, a method based on melt-kneading of propylene-α-olefincopolymer polymerized by using a Ziegler-Natta catalyst such astitanium-supported catalyst or commercially available ethylene-propylenerubber and crystalline polypropylene can be exemplified. As the methodfor continuously polymerizing crystalline polypropylene andpropylene-α-olefin copolymer by multi-step polymerization, for example,a method comprising producing propylene homopolymer in the first step,and producing propylene-α-olefin copolymer in the second step byutilizing a plurality of polymerization reactors can be exemplified.This continuous polymerization method is preferred, because it can beperformed at a lower cost compared with the aforementioned melt-mixingmethod, and can produce a polypropylene-based resin material where thepropylene-α-olefin copolymer is uniformly dispersed in the crystallinepolypropylene, and it is suitable for stably realizing the desiredquality (tensile elongation along the TD direction, preventing thegeneration of fibers, good transparence).

As the polypropylene-based resin material of the present invention,particularly preferred are those produced by the aforementionedcontinuous polymerization method so that the resulting material shouldhave the MFR ratio of the crystalline polypropylene to that of thepropylene-α-olefin copolymer (MFR of the crystalline polypropylene/MFRof the propylene-α-olefin copolymer) of 10 or less, more preferably inthe range of 0.1-5. By selecting the MFR ratio within this range, thepropylene-α-olefin copolymer can be uniformly and finely dispersed inthe crystalline polypropylene, and the copolymer particles can haveelongated form with an aspect ratio higher than a certain level afterthe uniaxial orienting. This provides a polypropylene-based orientedfilm that shows good straight cuttability along the MD direction, andtensile elongation along the TD direction further improved withoutdegrading the transparence.

Specifically, polypropylene-based resin materials having such an MFRratio can be produced by the methods mentioned in Japanese PatentUnexamined Publication Nos. 6-239918, 8-27238, and the like.

The MFR ratio can usually be calculated by measuring the MFR of thecrystalline polypropylene and the MFR of the propylene-α-olefincopolymer respectively, but when the polypropylene-based resin materialis continuously produced by the multi-step polymerization method (thecrystalline polypropylene is polymerized first, and then thepropylene-α-olefin copolymer is polymerized), the MFR of thepropylene-α-olefin copolymer cannot be directly measured. In such acase, the MFR of the propylene-α-olefin copolymer can be obtained fromthe MFR of the crystalline polypropylene, which can be directlymeasured, the MFR of the obtained polypropylene-based resin material,and the content of the propylene-α-olefin copolymer in thepolypropylene-based resin material according to the following equation:${\log \left( {MFR}_{RC} \right)} = \frac{{\log \left( {MFR}_{whole} \right)} - {\left( {1 - {W_{RC}\text{/}100}} \right){\log \left( {MFR}_{PP} \right)}}}{W_{RC}\text{/}100}$

MFR_(RC): MFR of propylene-α-olefin copolymer

MFR_(whole): MFR of polypropylene-based resin material

MFR_(PP): MFR of crystalline polypropylene

W_(RC): Content of propylene-α-olefin copolymer in polypropylene-basedresin material

(2) Film-forming Material of the Present Invention

While the film-forming material of the present invention is mainlycomposed of the aforementioned polypropylene-based resin material, itmay further contain additives conventionally used for polyolefin-basedfilm materials, for example, antioxidant, neutralizer, weathering agent,inorganic filler, lubricant, anti-blocking agents, antistatic agent andthe like.

Examples of the antioxidant include, for example, phenol compoundantioxidants such astetrakis[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane,2,6-di-t-butyl-4-methylphenol,n-octadecyl-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionat e, andtris(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate; phosphorus-containingantioxidants such as tris(2,4-di-t-butylphenyl) phosphite,tris(nonylphenyl) phosphite, distearylpentaerythritol diphosphite, andtetrakis(2,4-di-t-butylphenyl)-4,4′-biphenylenediphosphonite, and thelike.

Examples of the neutralizer include, for example, salts of higher fattyacid such as calcium stearate; examples of the inorganic filler and theanti-blocking agents include, for example, calcium carbonate, silica,hydrotalcite, zeolite, aluminum silicate, magnesium silicate and thelike; examples of the lubricant include, for example, higher fatty acidamides such as stearic acid amide and the like; and examples of theantistatic agents include, for example, fatty acid esters such asglycerin monostearate and the like.

While the amounts of these additives may be suitably selected dependingon the intended use of the film and the like, they are preferably usedin an amount of about 0.001-5% based on the whole film-forming materialin general.

The method for mixing the polypropylene-based resin material and theaforementioned additives is not particularly limited, and it can beperformed, for example, by mixing methods utilizing conventional mixingapparatuses including mixers provided with high-speed agitators such asHenschel mixer (trade name), ribbon blender and tumbler mixer and thelike (dry blend), as well as methods for pelletization utilizing aconventional single-screw extruder, double-screw extruder and the like.

(3) Uniaxial Orienting

The uniaxially oriented film of the present invention can be obtained byuniaxially orienting the aforementioned film-forming material. Theorienting can be performed by molding an unoriented sheet by the knownT-die cast method, water-cooled inflation method or the like, and thenorienting it by a known uniaxial orienting method such as the rollorienting method.

While the orienting ratio along the MD direction (longitudinal orientingratio) for the uniaxially oriented film of the present invention is notparticularly limited, it is, for example, 3-12 times, preferably 5-10times. An orienting ratio in such a range can afford an aspect ratio of100 or more for the ratio of the mean dispersed particle length to themean dispersed particle diameter along the film thickness direction in across section of the propylene-α-olefin copolymer domains along the MDdirection, and provide a film with improved tensile elongation withoutdegrading the transparence.

(4) Uniaxially Oriented Polypropylene-based Film

In the uniaxially oriented polypropylene-based film of the presentinvention, the propylene-α-olefin copolymer domains dispersed asparticles in the crystalline polypropylene have a mean dispersedparticle diameter along the film thickness direction of 0.10 μm or less,preferably 0.05 μm or less in a cross section along the MD direction.When the mean dispersed particle diameter is more than 0.10 μm, thetensile elongation along the TD direction is lowered, and transparenceis also degraded. On the other hand, the lower limit of the meandispersed particle diameter is not particularly defined, and it may behowever small so long as the copolymer domains can be observed. However,the mean dispersed particle diameter is preferably not less than 0.005μm.

The uniaxially oriented film of the present invention is alsocharacterized by the aspect ratio (L/D) of 100 or more, preferably 300or more, as for the ratio of the mean dispersed particle length (L) tothe aforementioned mean dispersed particle diameter (D) of the copolymerdomains in a cross section along the MD direction.

The relationship between the mean dispersed length (L) and the meandispersed particle diameter (D) is schematically shown in FIGS. 1(a) and(b). The mean dispersed particle diameter (D) in a cross section alongthe MD direction is the average of the particle diameter (breadth) ofthe dispersed particles along the film thickness direction when thecross section of the film along the MD direction is observed from theperpendicular direction to the MD direction (MD observation: edge view).The mean dispersed length (L) is the average of the length of thedispersed particles in the aforementioned MD observation.

According to the present invention, such fine and elongated copolymerdomains are uniformly dispersed in the matrix. This provides auniaxially oriented film excellent in tensile elongation along the TDdirection, and exhibiting transparence not degraded. An aspect ratio ofless than 100 is not preferred, because the film with such an aspectratio may degrade the tensile elongation along the TD direction andtransparence.

Although the upper limit of the aspect ratio is not particularlylimited, it is preferably about 700 when a length of one copolymerparticle is regarded as the particle diameter along the MD direction ofthe copolymer domain. However, the copolymer particles may be fused andconnected each other along the MD direction by the orienting along theMD direction. In this case, when the multiple fused copolymer particlesis considered as one copolymer domain, the particle diameter along theMD direction may be several times as large as the length of onecopolymer particle. The maximum aspect ratio of such a copolymer domainmay be several times that of the individual one copolymer particle,specifically 10 to 50 times. In this case, the aspect ratio may reach ashigh as around 1000 to 5000.

When a cross section along the TD direction of the film of the presentinvention is observed from the perpendicular direction to the TDdirection (TD observation: end view), the copolymer domains may be in aflattened form due to the uniaxial orienting along the MD direction. Insuch a case, the aspect ratio (L′/D) of the mean dispersed length (L′)to the mean dispersed particle diameter (D) along the film thicknessdirection in the cross section along the TD direction is preferably,while it is not particularly limited, about 80 to about 600. The TDobservation is schematically shown in FIG. 1(c).

According to the present invention, it was found for the first time thata film containing the copolymer domains which exhibited such a fine meandispersed particle diameter and such an aspect ratio as described abovewas excellent in the tensile elongation along the TD direction, and doesnot degrade transparence. Therefore, the film of the present inventionmay be a film obtained by any kind of method so long as the filmsatisfies the requirements concerning the particle diameter of thecopolymer domains. However, it can specifically be obtained by orientinga polypropylene-based resin material produced by the above-mentionedcontinuous polymerization method.

Particularly preferably, the film can be obtained by uniaxiallyorienting a polypropylene-based resin material being produced by thecontinuous polymerization method and having a ratio 10 or less of MFR ofthe crystalline polypropylene to that of the propylene-α-olefincopolymer to have an orienting ratio of about 3 to 12 times.

The thickness of the uniaxially oriented polypropylene-based film of thepresent invention is not particular limited, but it is preferably 10-100μm, more preferably 15-70 μm, in view of the film-forming property ofthe film.

The uniaxially oriented polypropylene-based film of the presentinvention is excellent in the straight cuttability along the MDdirection and the tensile elongation along the TD direction, does notgenerate fibers, and retains transparence. Therefore, it can bepreferably used as a material for packaging heavy contents, material forpackaging foodstuffs such as sandwiches and rice balls, and the like.

The uniaxially oriented polypropylene-based film of the presentinvention can also be used for a multilayer film comprising two or morelayers, which can be prepared by laminating one or more films made ofother resins on one or both sides of the film of the present invention.The other resins used for such a laminated film are not particularlylimited, and various resins can be used depending on the purpose of thefilm. For example, when a layer composed of heat adhesive resin such aspropylene-α-olefin copolymer having a low melting point is provided onthe uniaxially oriented film of the present invention, it can be used asvarious package materials. As production methods of such a multilayerfilm, the inline laminating method ,the co-extrusion method and thelike, which are performed during the film molding, as well as the drylaminating method and the like where the lamination is performed afterthe film molding, can be utilized.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 includes schematic views representing the relationship betweenthe mean dispersed length (L) and the mean dispersed particle diameter(D) in a cross section along the MD direction. FIG. 1(a) is aperspective view of the film, FIG. 1(b) is a view of the MD observationrepresenting a cross section along the MD direction, and FIG. 1(c) is aview of the TD observation representing a cross section along the TDdirection.

FIG. 2 is an electron microscope photograph (magnification: ×7500) whichshows the particle state of copolymer domains along the MD direction inthe uniaxially oriented film obtained in Example 3.

FIG. 3 is an electron microscope photograph (magnification: ×7500) whichshows the particle state of copolymer domains along the TD direction inthe uniaxially oriented film obtained in Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be further explained more specificallyhereinafter with reference to the following examples, but the presentinvention is not limited by these examples.

EXAMPLES 1-6 AND COMPARATIVE EXAMPLES 1-4 (1) Production of Film-formingMaterials

To the polypropylene-based resin materials shown in Table 1 were eachadded 0.1% by weight of tetrakis[methylene-3-(3+,5′-di-t-butyl-4′-hydroxyphenyl) propionate]methane as aphenol compound antioxidant, 0.1% by weight oftris(2,4-di-t-butylphenyl)phosphite as a phosphorus-containingantioxidant, and 0.1% by weight of calcium stearate as a neutralizer,based on the whole film-forming material. These were blended by aHenschel mixer (trade name), melt-kneaded and pelletized by asingle-screw extruder (aperture of 40 mm) to afford a film-formingmaterial.

The polypropylene-based resin materials used in these examples wereobtained by the continuous polymerization method where crystallinepolypropylene was polymerized in the first step, and propylene-α-olefincopolymer (propylene-ethylene copolymer) was polymerized in the secondstep. However, the polypropylene-based resin material used inComparative Example 3 was obtained by melt-kneading of commerciallyavailable ethylene-propylene rubber (propylene content: 27% by weight,produced by Japan Synthetic Rubber Co., Ltd) and crystallinepolypropylene.

The values of MFR of the polypropylene-based resin materials and that ofthe crystalline polypropylenes shown in Table 1 were determinedaccording to JIS-K-7210 under the conditions of test temperature of 230°C. and test load of 21.18 N.

(2) Production of Uniaxially Oriented Films

Each film-forming material in the form of pellets obtained above wasmelt-extruded at 260° C. by using an extruder equipped with a T-die, andcooled by a cooling roller at 30° C. to be solidified to obtain anunoriented sheet. This sheet was preheated at 90° C. by a preheatingroll, and oriented 5 times along the longitudinal direction (MDdirection) between rolls at 100° C. to form a uniaxially oriented filmwith a thickness of 30 μm.

(3) Evaluation

Various physical properties of the obtained uniaxially oriented films,i.e., mean dispersed particle diameter and aspect ratio in a crosssection along the MD direction of the copolymer domains in the films,transparence of the films (haze), Young's modulus along the MDdirection, and tensile breaking elongation along the TD direction areshown in Table 1. The methods for evaluating these physical propertiesare as follows.

(a) Particle diameter and aspect ratio in cross section along MDdirection of copolymer domains

Uniaxially oriented film was cut along the perpendicular directions tothe MD direction and the TD direction respectively, dyed in vapor phasewith a ruthenium compound (RuO₄) for 48 hours, and then cut into pieceshaving a thickness of about 100 nm with a diamond knife using anultramicrotome to prepare ultrathin sections. The obtained ultrathinsections were observed by using a transmission electron microscope(tradename: JEOLEM 100CX) at magnifications of 10,000 and 30,000, eachof mean dispersed length of the copolymer domains and mean dispersedparticle diameter along the film thickness direction in the crosssection along the MD direction was obtained through statisticalprocessing of the electron microscope photograph, and aspect ratio wascalculated from them.

(b) Tensile breaking elongation (TD direction)

Degree of tensile breaking elongation (tensile elongation) along the TDdirection of each uniaxially oriented film was measured according toASTM-D-882, and used as an index of difficulty for tearing along the MDdirection. A larger value means that the film is more difficult to betorn along the MD direction.

(c) Haze

Haze of uniaxially oriented film (unit: %) was measured according toASTM-D-1003, and used as a parameter of transparency. A smaller valueindicates better transparency.

(d) Young's modulus (MD direction)

According to ASTM-D-523, Young's modulus along the MD direction ofuniaxially oriented film was measured, and used as a parameter ofrigidity. A larger value indicates that the film had higher rigidity.

TABLE 1 Example Comparative Example 1 2 3 4 5 6 1 2 3 4 (1)Polypropylene-based resin Crystalline polypropylene content (wt %)  83.5 76.9  67  67  67  57  96  80  80  100 Propylene content in crystallinepoly-  100  100  100  99.7*¹  100  100  100  100  100  100 propylene (wt%) Copolymer*² content (wt %)  16.5  23.1  33  33  33  43   4  20  20  0 Propylene content in copolymer (wt %)  64  60  60  62  55  64  36 50  27   0 MFR of polypropylene-based resin (g/   2.8   3.4   0.5   3.1  8   0.4   2.8   9.4   5.5   2 10 min) MFR of crystalline polypropylene(g/10 min)   3.2   5   0.5   3.1  11.5   0.4   3  22   9.2   2 Ratio ofMFR*³   2   2   1   1   3   1   2  75  13.1 * (2) Various physicalproperties of film Mean dispersed particle diameter of   0.04   0.04  0.035   0.035   0.04   0.04   0.04   0.15   0.27 — copolymer (μm)Aspect ratio of copolymer (L/D) >300 >300 >300 >300 >300 >300 >300  35 30 — Haze (%)   8  11  12  10  15  19  18  68  74   4 Young's modulus(MPa) 2200 1900 1500 1400 1300 1100 3000 2100 2300 3100 Tensile breakingelongation (TD, %)  490  720 1070  950  760 1280   7  360  260   5*¹Propylene/ethylene copolymer *²Propylene/ethylene copolymer *³Ratio ofMFR = MFR of crystalline PP/MFR of copolymer

The electron microscope photograph (magnification: ×7500) of theultrathin section, used for obtaining the mean dispersed particlediameter and the mean dispersed length of the copolymer domains in across section along the MD direction of the uniaxially oriented filmobtained in Example 3, was shown in FIG. 2. FIG. 2 shows a photographobtained by observing from the perpendicular direction to the MDdirection (MD observation). An electron microscope photograph obtainedby observing from the perpendicular direction to the TD direction (TDobservation) is shown in FIG. 3. FIG. 2 and FIG. 3 show electronmicroscope photographs representing particle state of the copolymerdomains in the aforementioned film along the MD and TD directions.

As seen from the results shown in Table 1, the films of Examples 1-6showed high tensile elongation along the TD direction, and were alsoexcellent in rigidity (Young's modulus) and transparence (haze).Moreover, as seen from FIGS. 2 and 3, the uniaxially oriented film ofthe present invention had elongated copolymer domains dispersed finelyand uniformly.

On the other hand, the film of Comparative Example 1 had a low contentof the propylene-α-olefin copolymer in the polypropylene-based resinmaterial and sufficient tensile elongation could not be obtained. InComparative Examples 2 and 3, obtained were only films showing too largemean dispersed particle diameters of the propylene-α-olefin copolymerand having low aspect ratios, poor transparence, and low tensileelongation along the TD direction. In Comparative Example 4, thepolypropylene-based resin material composed only of crystallinepolypropylene was used, and thus a film having sufficient tensileelongation could not be obtained.

INDUSTRIAL APPLICABILITY

The uniaxially oriented film of the present invention has good straightcuttability along the MD direction, and generates few fibers when it istorn, and it is excellent in tensile elongation and strength along theTD direction, and retains transparence.

What is claimed is:
 1. A uniaxially oriented polypropylene film formedfrom a film-forming material which comprises a polypropylene resinmaterial consisting of 40 to 95% by weight of crystalline polypropylene,and 60 to 5% by weight of propylene-α-olefin copolymer dispersed asparticles in the crystalline polypropylene, wherein the crystallinepolypropylene is a propylene homopolymer or a propylene-α-olefincopolymer having propylene unit content of 90% or more by weight, thepropylene-α-olefin copolymer dispersed as particles contains 20-80% byweight of propylene polymerization units, the polypropylene resinmaterial is obtained by continuously polymerizing the crystrallinepolypropylene and the propylene-α-olefin copolymer dispersed asparticles by multi-step polymerization, and the particles of thecopolymer have an aspect ratio (L/D) of mean dispersed particle length(L) to mean dispersed particle diameter along the film thicknessdirection (D) of 100 or more in a cross section of the film along the MDdirection, and the mean dispersed particle diameter of 0.10 μm or less.2. The uniaxially oriented polypropylene of claim 1, wherein a ratio ofMFR of the crystalline polypropylene to that of the propylene-α-olefincopolymer dispersed as particles, which represents MFR of thecrystalline polypropylene/MFR of the propylene-α-olefin copolymerdispersed as particles, is 10 or less.
 3. The uniaxially orientedpolypropylene film of claims 1 or 2, wherein the propylene-α-olefincopolymer dispersed as particles comprises a propylene unit and anethylene unit.
 4. The uniaxially oriented polypropylene film of claims 1or 2, wherein the crystalline polypropylene is a propylene homopolymeror a propylene-ethylene copolymer, and the propylene-α-olefin copolymerdispersed as particles is a propylene-ethylene copolymer.
 5. Auniaxially oriented polypropylene film formed from a film-formingmaterial which comprises a polypropylene resin material consisting of 40to 95% by weight of crystalline polypropylene, and 60 to 5% by weight ofpropylene-α-olefin copolymer dispersed as particles, and has a ratio of10 or less of MFR of the crystalline polypropylene to that of thepropylene-α-olefin copolymer, wherein the film is uniaxially orientedalong the MD direction so that the oriented ratio is 3 to 12 times, andwherein the particles of the copolymer have an aspect ratio (L/D) ofmean dispersed particle length (L) to mean dispersed particle diameteralong the film thickness direction (D) of 100 or more in a cross sectionof the film along the MD direction, and the mean dispersed particlediameter of 0.10 μm or less.
 6. The uniaxially oriented polypropylenefilm of claim 5, wherein the propylene-α-olefin copolymer comprises apropylene unit and an ethylene unit.
 7. The uniaxially orientedpolypropylene film of claim 5, wherein the crystalline polypropylene isa propylene homopolymer or a propylene-ethylene copolymer, and thepropylene-α-olefin copolymer is a propylene-ethylene copolymer.